Chlorodiazaphospholines as intermediates in the synthesis of 1,2-dihydro-2-alkenyl-3H-pyrazol-3-ones and 2-pyrrolylacetates^†

New derivatives of 1,2-dihydro-2-alkenyl-3H-pyrazol-3-ones 6 and 7 have been synthesized at room temperature by a one-pot two-step reaction of PCl₃, a ketone methylhydrazone, and a β-keto ester. With ketone methylhydrazones bearing at least a phenyl group in the α-positions to the CN bond and in the second step β-keto esters, such as propionyl- or butyrylacetates, we obtained 2-pyrrolylacetates 8 as unexpected products together with pyrazolones 7. The ratio of the two products depends on the nature of the groups in the α-positon to the CN bond. A chlorodiazaphospholine 1 is the key intermediate of this new reaction, and a plausible mechanism of formation of the azaheterocycles is reported.